In humans, loss of functional small bowel surface area due to etiologies such as surgical resection, vascular injury or inflammatory bowel disease leads to an adaptive response in the remnant intestine. The residual gut epithelium becomes markedly hyperplastic and displays increased villus height and crypt depth, leading to enhancement o nutrient absorption. The ability of the gut to renew itself is limited, thus many patients who have sustained the loss of large amounts of bowel depend upon parenteral nutrition for survival. The long term goal of this research is to understand the molecular regulation of the adaptive response to permit the design of specific clinical regimens to enhance this process, eliminate the need for parenteral nutrition in some patients, and improve the quality of life of others. The cellular and molecular mechanisms underlying the adaptive response are unknown. Attempts to define these mechanisms have focused on administering growth factors, hormones and nutrients and assessing crypt cell proliferation. Few genes specifically regulated in adaptation have been identified. Also, the state of differentiation and the functional contribution of villus-associated enterocytes are unresolved. To map the cellular and molecular pathways that lead to gut epithelial adaptation, an analysis of both villus and crypt cells responses is required. These issues are being' addressed by cloning and characterizing genes differentially regulated during adaptation. The enterocytic response to adaptation is being systematically examined using these clones and a well-defined set of genes as markers of cellular differentiation. An established rat model of adaptation will be used to address these hypotheses: (i) Novel, previously uncharacterized intestinal genes, such as we have cloned, act as mediators, modifiers and/or are specific phenotypic markers of epithelial cell proliferation and differentiation in adaptation. These genes are predominantly expressed in crypt cells in early adaptation, but may also be transcribed in villus epithelium. (ii) Along with crypt cells, villus enterocytes respond rapidly and specifically to loss of small bowel Surface area and contribute significantly to the adaptive response. To test these hypotheses, the specific aims are: 1. To characterize genes isolated using subtractive hybridization and RNA differential display that are differentially expressed in the adapting small intestine following 70% resection of the rat small bowel. 2. To examine the enterocytic response post-resection using selected enterocyte specific clones and well- characterized genes that are sensitive markers of gut epithelial differentiation. Analysis of the response of crypt cells and enterocyte may offer new approaches to enhancing mucosal response to disease or resection.